Shadow lines on faces on mesh
As LukeD commented, meshes should consist of quads of roughly the same size. A standard way of rendering faces is to take a color assigned to a given vertex of the face and linearly interpolate the color across the face to the next vertex. So if your faces are not roughly uniform in size, you can get these long shadows stretching across the surface of your model.

Basically, the lights in your blender editor illuminate the vertices. If one vertex of a face is very far from another vertex, the vert farther from the light will be assigned a darker color, and the "color gradient" across the long face is very noticeable (because it's just evenly blending the colors of the vertices across the face). If you add more polys, there will be more vertices to illuminate, which will give more accurate coloring.